Electrostatic printers and copiers commonly use an electrostatic writing head to form a latent image of electrical charges onto a recording medium such as a flexible paper web. The paper web carrying the latent image is then directed to a liquid toning applicator which deposits oppositely charged toner particles onto the paper web, thereby developing the latent image.
Generally, there are two designs for liquid toning applicators: vacuum-type or pressure-type. The vacuum-type applicators employ a liquid pump to draw toner from a reservoir into and through one or more channels located in the face of the applicator to come into contact with the flexible paper web. The web's flexibility serves to seal the face of the applicator thus permitting the pump to create a vacuum in the channels. The vacuum draws toner into and through the channels. The outlet of the toner pump returns the spent toner to the reservoir. In this manner, a continuous recirculation of the toner occurs.
Despite the inherent advantage of leak protection, suction-type applicators have limited application. These applicators are plagued by slow toning speeds that result from the paper web coming into contact with the toner only in small channels. The channels must be very small to prevent the paper web from being pulled down into them. As the speed of the web passing over the applicator is increased, additional channels are required which leads to increased sliding friction, a phenomena sought to be avoided. Moreover, narrow channels require a greater vacuum to maintain toner flow. They also increase sliding friction by adding to the downward force on the paper web. In addition, this design is unsuitable for creating pictorial images.
The speed and image quality limitations of vacuum-type applicators brought about the first pressure-type toning applicator, in which a wetted roller is rotated against the latent image bearing sheet. A scraper blade removes the excess spent toner from the roller prior to re-wetting with toner and again contacting the image. This method provides uniform toning as well as very low sliding friction. The toner cascading down the scraper blade flows in an unconstrained or uncontained manner and has to be collected with a full width funnel or gutter, similar to collecting rainwater by a roof gutter. This gutter is hard to clean by simple rinsing compared to the small vacuum slits of the previous art.
The small slits of the vacuum system can be scoured by simply passing clean fluid through them at high speed. With the roller system, running clear fluid through the toning system can effect cleaning but it is very slow, in fact, too slow to permit a single applicator to be used for multiple colors. Thus, with prior art toning methods, both vacuum and pressure, it is not possible to achieve high quality and high toning speed in a cleanable toning system. Therefore, it is not possible to achieve the cost advantages of a single applicator design in a high speed, high quality printer. This fact led to the development of another type of pressure toning system, namely the positive air-pressure design. This new design permits high quality and speed in a readily cleanable system.
The positive air-pressure system, described in U.S. Pat. No. 5,268,721 to Day, assigned to the assignee of the present invention, uses pressurized air to confine the toner rather than suction. This permits the toner to be pumped through the channels of an applicator rather than drawn through by suction. The web is not drawn tightly against the face of the applicator and high friction is avoided. Furthermore, the toner flow is totally contained and high speed cleaning is possible in contrast to the open-flow of the roller system. Since there is no suction, the channels facing the web can be arbitrarily large and this permits high speed toning as well.
As disclosed in U.S. Pat. No. 5,268,721 a pressurized air channel completely surrounds the wet area of the applicator. The fact that the air pressure is higher than the fluid pressure assures that toner cannot escape against the higher air pressure. In the prior vacuum system, ambient or room air pressure is higher than the liquid pressure and leakage is prevented. Since the toner is confined in the positive air-pressure applicator, no funnel or gutter is needed and high speed cleaning is possible. A planar backing member behind the paper web, i.e. on the other side of the web, is used to keep the web flat against the applicator face.
U.S. Pat. No. 5,296,899 to Day, also assigned to the assignee of the present invention, discloses a positive air-pressure applicator with a segmented backplate which flexibly retains the paper web in close proximity to the surface of the applicator so that excess air does not leak from between the applicator surface and the paper web. In one embodiment, a backing plate segment is elastically supported by a cross member via a compressed spring. In an alternate embodiment, a leaf spring is riveted to the cross member and supports the backing plate segment. The backing plate is formed from a plurality of segments. Each segment is independently supported by the cross member via a compressed spring. In this manner, the backing plate may conform to the irregularities in the surface of the applicator and, thereby, avoid excess air leakage.
U.S. Pat. No. 5,231,455 to Day discloses a method and apparatus for reducing the effective common volume of an applicator that is shared with a plurality of reservoirs containing toners of different colors. A pump is employed for applying toner to the paper web and washing. An air-blower is employed to purge the applicator of toner.
A problem encountered with the aforementioned inventions is the leakage of toner while purging the toner from the applicator. What is needed is a graphics quality applicator that can be cleaned by passing a dispersant through the unit while avoiding toner leakage.